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whats the min thickness before bmw does a rotor replacement as part of a brake job...im trying to figure out if the rotors were replaced too as part of my brake service....i see no part numbers for rotors listed and then it says replaced rear pads 2mm replaced front pads 2mm, ft discs 23.55mm rear disc 20.9mm.... im not sure if that means they replaced the rotors or that they are just listing what they mic'd

Not sure about the rears, other than being smaller diameter, I would guess the spec. thickness is 2 -3 mm. less than the fronts.
Let's say 2.7mm., so that would make it 20.80mm. putting yours just on the safe side.

The minimums are normally stamped on the rotors somewhere, so they presumably verified that they did not need replacing.

My rear pads are on their final days at 65k miles....I'm replacing them soon. Revisiting this thread, does anybody know the specs for the rotors? I want to make sure is safe just changing the pads. I've hear people saying the rotors life outlast the pads? And they are replacing rotors every two pad sets. What's the concensus here?

I'd recommend going to auto zone and buying the All-Data CD for your vehicle. Well worth the $25 ish dollar investment, it will have torque specs, thickness specs, etcc..but it goes into thousandths of an insh so you'd NEED a micrometer if you're trying to do a DIY - 0.3 - 1.3 range. Random specs are hard to find laying around on the internet.

It is true that rotors will usually outlast a single set of pads. What concerns me about the original poster's rotors (and that post being from February is long out of date) is that while they were currently in spec, they will be far under spec by the time a whole new set of pads are used. Rotors are fairly cheap, and it is little more work to change a rotor when you're already changing pads, so if there is any doubt I would go ahead and change them, especially in the front (rear brake wear is considerably less than the front)

It is true that rotors will usually outlast a single set of pads. What concerns me about the original poster's rotors (and that post being from February is long out of date) is that while they were currently in spec, they will be far under spec by the time a whole new set of pads are used. Rotors are fairly cheap, and it is little more work to change a rotor when you're already changing pads, so if there is any doubt I would go ahead and change them, especially in the front (rear brake wear is considerably less than the front)

+1 on that. The rotors are easy to change. Just do it or you will regret it later!